In the past, a braking device is known, which comprises a non-movable body, a movable body capable of rotating inside of the non-movable body, a magnetorheological fluid filled between the movable body and the non-movable body, and a coil configured to generate a magnetic field applied to the magnetorheological fluid.
In this kind of braking device, a shearing stress of the magnetorheological fluid is increased when the magnetic field is applied to the magnetorheological fluid, and the braking device, by making use of the increase of the shearing. stress of the magnetorheological fluid, decelerates the rotation speed of the movable body or stops the rotation of the movable body. The magnetic field applied to the magnetorheological fluid is generated by passing an electric current through the coil.
However, in order to cause this kind of braking device to generate a larger braking force, the shearing stress of the magnetorheological fluid is required to be generated at a position further away from the center of rotation of the movable body, and the braking device is required to have a larger surfaces facing each other with the magnetorheological fluid interposed therebetween. For example, Japanese Patent No. 4695835 discloses a braking device using a magnetorheological fluid, but this braking device does not have such structure. Therefore, it used to be difficult to generate a large braking force.